Unfo I have no wiring diagrams, but as far as I have read they are told to be heated all the time when acc is on. Would have to check it tomorrow on my way to work. If it is like that it would be very easy to do, piece of wire and one relay.

Well I'm not so sure, for my curiosity I will check tomorrow morning. Honestly speaking both options are far from being perfect - heating them constantly makes no sense in summer, heating them together with windscreen means they are turned off after about 5 minutes and what next? Nobody is using heated windscreen all the time, in winter conditions it means that after few minutes washing fluid could freeze anyway.

I tend to find that the only one that freezes is the rear washer due to a longer pipe, not sure what path it takes through the car but it must be exposed somewhere.
The front ones tend to be ok due to engine heat, it has to be pretty extreme to freeze them.
You could try adding some cheap vodka to the washer bottle, that would work better than washer fluid.

Yeah they defo come on with the heated windscreen, and the heated mirrors come on with the heated rear window. I think the theory is that you only put the heated windscreen on when its frozen or really cold/misted up. This means the washers will de-frost at the same time. After the washers are warm, the engine heat will then keep them from freezing so you wouldn't need to have them heated all the time. Also the anti freeze will help too once they are warm and been used a bit.